Extrarenal water loss and antidiuretic hormone

Physiology of sweat gland function: The roles of sweating and sweat composition in human health

The purpose of this comprehensive review is to: 1) review the physiology of sweat gland function and mechanisms determining the amount and composition of sweat excreted onto the skin surface; 2) provide an overview of the well-established thermoregulatory functions and adaptive responses of the sweat gland; and 3) discuss the state of evidence for potential non-thermoregulatory roles of sweat in the maintenance and/or perturbation of human health. The role of sweating to eliminate waste products and toxicants seems to be minor compared with other avenues of excretion via the kidneys and gastrointestinal tract; as eccrine glands do not adapt to increase excretion rates either via concentrating sweat or increasing overall sweating rate. Studies suggesting a larger role of sweat glands in clearing waste products or toxicants from the body may be an artifact of methodological issues rather than evidence for selective transport. Furthermore, unlike the renal system, it seems that sweat glands do not conserve water loss or concentrate sweat fluid through vasopressin-mediated water reabsorption. Individuals with high NaCl concentrations in sweat (e.g. cystic fibrosis) have an increased risk of NaCl imbalances during prolonged periods of heavy sweating; however, sweat-induced deficiencies appear to be of minimal risk for trace minerals and vitamins. Additional research is needed to elucidate the potential role of eccrine sweating in skin hydration and microbial defense. Finally, the utility of sweat composition as a biomarker for human physiology is currently limited; as more research is needed to determine potential relations between sweat and blood solute concentrations.

The effect of antidiuretic hormone on human sweating

1. Changes in insensible perspiration and sweating were followed in normal subjects by continuously monitoring total body weight loss in environmental temperatures of 18, 29 and 37 degrees C.2. Pharmacological doses of ADH had no effect on cutaneous water loss at 18 degrees C.3. Pharmacological doses of ADH are capable of increasing the rate of cutaneous water loss in human subjects who are close to or above the thermal sweating threshold.4. Physiological doses of ADH had no effect on cutaneous water loss in either cool or hot environments.5. At normal rates of secretion in the body, ADH probably does not influence human sweat secretion.

Selective blockade of vasopressin V2 receptors reveals significant V2-mediated water reabsorption in Brattleboro rats with diabetes insipidus

BACKGROUND

In a previous study we observed that acute administration of the selective antagonist of vasopressin (AVP) V2 receptors, SR 121463A (SR), aggravated the symptoms of diabetes insipidus (DI) in homozygous Brattleboro rats (an AVP-deficient strain). The present study investigates in more details the acute and chronic effects of SR in DI rats.

METHODS AND RESULTS

In experiment A, different groups of rats received acute i.p. injections of SR (0.001-10 mg/kg) or vehicle alone, and urine was collected for the next 24 h. SR dose-dependently increased urine flow rate and decreased urine osmolality with no significant change in solute excretion, thus confirming a pure 'aquaretic' effect. In experiments B and C, the chronic effects of orally administered SR were evaluated over 8 days in Brattleboro DI rats (experiment B, 1 mg/kg/day) and in adult Sprague-Dawley rats with normal AVP secretion (experiment C, 3 mg/kg/day). In DI rats, the aquaretic effects of SR persisted with the same intensity over the 8 days. In Sprague-Dawley rats, SR induced a sustained, stable aquaretic effect and also increased non-renal water losses, suggesting an effect of AVP on water conservation in extrarenal sites. Because oxytocin (OT) synthesis is elevated in DI rats and OT is known to bind to V2 receptors, we evaluated the antidiuretic effects of OT in DI rats in experiment D. Chronic infusion of OT (3 microg/kg/h, i.p.) induced a marked antidiuresis, and acute SR (1 mg/kg) in OT-treated DI rats completely abolished this antidiuretic effect, thus indicating that it was due to binding of OT to V2 receptors.

CONCLUSION

(i) SR is a potent orally active aquaretic and induces stable effects during 1 week in rats with or without endogenous AVP secretion. (ii) Significant V2 receptor-mediated water reabsorption occurs in collecting ducts of Brattleboro DI rats because their usual urine osmolality is about twofold higher than the minimum observed during SR-induced maximum diuresis. (iii) This V2 agonism could be mediated in part by OT binding to V2 receptors. Small amounts of endogenous AVP, known to be produced by adrenal and testis in DI rats, could also contribute to this V2 agonism, as well as a possible constitutive activation of the V2 receptors. (iv) In normal rats, AVP probably reduces water losses through extrarenal sites, probably the lungs.

ADH and thermal sweating

Sweating responses to heat exposure were compared in healthy subjects pretreated with pitressin or alcohol and in the control group. Between the three groups, there were no consistent differences in the rate of sweating expressed both as a total body weight loss during 2-h heat exposure and in mg of sweat per skin area covered by a paper disc. Likewise, there were no differences in the sweat osmolality or electrolyte concentration. There was also no evidence of inverse correlation between plasma ADH level and rate of sweat secretion or its concentration when pooled data of all subjects were analyzed. It was concluded that ADH did not substantially affect thermal sweating in men.